1. Field of the Invention
The invention pertains to the fabrication, by vapor phase deposition, of thin layers or films of a monocrystalline, polycrystalline or amorphous material on a substrate having an identical or different nature.
The invention also covers the making of monolithic electronic components, notably in integrated circuits (ICs), by means of a method such as this for the fabrication of thin layers.
The method described according to the invention particularly makes it possible to achieve a controlled variation in the composition and/or doping profiles of such layers in a way that is directed, notably in a lateral direction parallel to the surface of the substrate, unlike in usual techniques of epitaxy or of doping which create profiles of modulation in the composition or doping that vary essentially in a direction that is perpendicular to the surface of the substrate.
2. Description of the Prior Art
More precisely, in the field of micro-electronics and III-V semiconductors in particular, a basic method in the fabrication of discrete devices or of integrated circuits consists in the obtaining, by epitaxial growth, of layers doped at different levels on semiconductor substrates. Several methods of epitaxy are known such as, notably, vapour phase epitaxy (VPE) and its variants such as,, notably, metalorganic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE). In current practice, a common feature of all these methods is that the composition profile achieved during one and the same deposition is uniform in a given plane parallel to the surface of the substrate and can vary only in a direction perpendicular to the surface of the substrate. This is sufficient, and even indispensable, in most cases in the planar technology of ICs. There are, however, cases where it would be advantageous to be able to cause variation in the composition profile of active zones of devices in a direction parallel to the plane of the surface of the substrate.
A typical example is the making of a planar laser wherein the composition of the various layers varies in a direction parallel to the plane of the semiconductor surface. FIG. 1A shows the drawing of a simple laser made of GaAlAs/GaAs/GaAlAs, obtained in a known way by growth perpendicular to the plane of the substrate 10. FIG. 1B shows the arrangement of the layers of a planar laser having a direction of growth that is parallel to the plane of the substrate 10, and having a structure which has, therefore, been rotated by 90.degree. with respect to the known laser of FIG. 1A.
Now, up till the present time, there has been no method for making the structure of FIG. 1B.
A first aim of the invention is to provide a method for the fabrication of devices with directed modulation of the composition profile and, notably, of a planar laser such as this.
In the field of micro-electronics today, another basic method in the making of discrete devices or integrated circuits consists in the defining of active zones in the semiconductor substrate by doping. Several doping methods are known, notably ion implantation, vapor phase epitaxy or thermal diffusion. In current practice, a common feature of all these methods is that the doping profile achieved during one and the same step is uniform in a given plane parallel to the surface of the substrate. This is sufficient, and even indispensable, in most cases in the planar technology of ICs. There are, however, cases where it would be advantageous to be able to make the doping profile of an active zone vary in a direction parallel to the plane of the surface of the substrate, for example making a hyperabrupt plane varicap diode with laterally graduated doping profile as shown in FIGS. 2 and 3.
A known method to create laterally variable doping profiles is provided by focused ion beams. This approach enables implantation without mask with a lateral resolution of the order of 0.1 .mu.M (see J. Melngaisis, J. Vac. Sci. Technol., B5(2), 469, 1987, on the technology and applications of focused beams). Several published patent documents are based on this idea (see inter alia the French publications 8518985, 8518984 and 8519494). This approach is an attractive one but suffers from a major drawback, related to the low current of emission of the ion sources: the very slow writing speed. Moreover, it calls for the use of focused ion beam machines which are at a stage of development that is still, substantially, prior to that of industrial application. Furthermore, a thermal annealing step for the activation of the dopants is needed, as in the case of standard ion implantation.
A second aim of the invention is to provide a method for the directed (notably lateral) modulation of the doping profile, according to a fairly simple principle, with a high lateral definition, this method being above all capable of being performed with means commonly used in the micro-electronics industry (FIG. 3).
Finally, it is also possible to consider applications where there is need for a modulation of both the doping profile and the composition profile. Devices such as this have already been proposed in the literature on the subject. These devices are for example, Gunn oscillators with (Al:Ga)As heterojunction cathode (see A. Al-Omar et al, Proceedings IEEE/Cornell Conference on Advanced Concepts in High Speed Semiconductors Devices and Circuits in High Speed Semiconductor Devices and Circuits, August 1987, p. 365). A structure such as this is shown in FIGS. 4B, 4C facing a conventional Gunn diode with two abrupt fronts 34, 35 of the doping profile 36, for comparison (FIG. 4A). The known structure of FIGS. 4B and 4C is a "vertical" structure, namely a structure for which the doping profile 31 and composition profile 32 (note the graduated modulation 33 of the profile 32 on the cathode side) vary in a direction perpendicular to the plane of the substrate.
A third essential aim of the invention is therefore a method enabling the making of this structure with a modulation of both the composition and the doping, in a direction that is not perpendicular to the surface of the substrate, notably in a lateral way to obtain a planar type of technology and to be capable of integrating this device into a circuit.
With respect to the above example, it must be emphasized that the invention makes it possible to achieve laterally graduated doping and/or composition profiles as well as the lateral high definition required by this device.